Sex determination accuracy using the human sacrum in a Central European sample

The sacrum is often used for sex estimation of human remains. However, reported sexual classification accuracies based on the sacrum produce highly discrepant estimates ranging from 54% to 98% depending on the method and reference sample. Here, we therefore aim to evaluate the sacrum's efficacy in differentiating males from females by comparing three different approaches in a homogeneous Central European sample (n = 58, 27 males and 31 females) obtained from the 19th century Weisbach collection. Specifically, we investigated the sacrum by 1) a qualitative visual inspection, 2) traditional linear metrics, namely, sacral width, corpus width, and the associated corporo-basal index, and 3) geometric morphometrics (GM) using a 3D configuration of 100 landmarks and semilandmarks. Classification accuracies for the qualitative approach ranged from 69% to 81%. The investigated quantitative methods based on linear dimensions led to comparable classification accuracies of 62% for sacral width and 78% for the corporo-basal index. However, absolute corpus width had a stronger discriminative power (86%), similar to the PC scores from shape space (87%). Unexpectedly, the GM approach, which considered the shape of the entire sacrum, did not classify better than linear variables. This was exceeded only when sacrum shape was combined with corpus width or another measure of size, like the natural logarithm of the centroid size, yielding a classification accuracy of 95%. The male and female group means differed mainly in the general height-to-width relationship and corpus-to-alae proportions, corroborating patterns previously described in the literature. Our results suggest that the sacrum is markedly less effective than the hipbone for sex determination, which is at odds with its central position within the pelvic girdle and thus its supposed obstetric relevance.